Pm fiber alignment

ABSTRACT

The polarization axes of the ends of two PM fibers are aligned in an automatic fiber splicer by first making a linear alignment of the fiber ends ( 1, 1′ ) using movable retainers ( 21 ) the same way as for conventional splicing. The fiber ends are rotated by rotatable fixtures ( 22 ) to capture images by a camera ( 9 ) and therefrom, in an image processing and analysis unit ( 15 ), as controlled by logical circuits ( 33 ) light contrast profiles are determined as functions of the angular position. From the light contrast profiles the polarization axes are determined and then they are aligned with each other. The images are captured of an area at and around the fiber ends as seen in an observation plane. This observation plane is taken to have such a position that the variation of the light contrast profiles is sufficiently large, this making the determination of the angular positions of the polarization axes have a sufficient accuracy, also for for example elliptical core fibers.

RELATED APPLICATIONS

This application claims priority and benefit from Swedish patentapplication No. 0200569-2, filed Feb. 26, 2002, the entire teachings ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to alignment of the fast and slow opticalaxes of the ends of two polarization maintaining (PM) fibers to befusion spliced to each other, in particular to alignment of ellipticalcore fibers having a relatively weak ellipticity.

BACKGROUND

Before splicing two optical fibers to each other, a proper mutualalignment of the fibers is essential, since this will minimize theoptical attenuation for light propagating in the fibers and through thesplice. In the particular case of aligning two PM fibers to each other,special consideration of the geometry of the fibers has to be made. Likeconventional fibers, commercially available PM fibers have a core regionand a surrounding cladding, the cladding having generally acircular-cylindrical outer surface. However, the distribution of therefractive index over a cross-section perpendicular to the longitudinalaxis of PM fibers is not circular-symmetric with respect to the fiberaxis unlike the conventional case.

For splicing PM fibers to each other an important issue is therefore toachieve a good angular alignment or azimuthal alignment, so that, fortwo PM fibers, regions of equivalent refractive indices are as close aspossible to each other at the two opposite fiber end faces, locatedclosely at each other, at which the fibers are to be spliced to eachother. Two basic methods are frequently used for the angular alignment,the so-called active and passive alignment methods. For the activealignment method, a highly polarized light source, a polarizationextinction ratio (PER) meter and an apparatus provided with opticalfiber rotators are needed. The PER is defined as the optical power ratioin dB form measured along two main optical axes. The angular alignmentcan be achieved by maximizing the value of PER while rotating one fiberend with respect to the other at the splicing point. A typical apparatususing the active method for angular alignment of PM fibers was disclosedin 1992, see U.S. Pat. No. 5,156,663, Oct. 20, 1992, for Keinichiro Itohet al.

The passive alignment method is performed locally at the splice pointwith the assistance of digital imaging techniques in an automated fusionsplicer. Several different techniques have been developed for passivelyaligning PM fibers. A method using an interference pattern to determinethe polarization axes of PM fibers was disclosed in 1994, see U.S. Pat.No. 5,323,225, Jun. 21, 1994, for Richard B. Dyott et al. A method usingthe photoelastic effect to determine the polarization axes of PM fiberswas disclosed in 1995, see U.S. Pat. No. 5,417,733, May 23, 1995, forLaurence N. Wesson. Methods of intensity profile analysis have also beenproposed, e.g. the fiber side-view method, see H. Taya, K. Ito, T.Yamada and M. Yoshinuma, “New splicing method for polarizationmaintaining single mode fibers,” Conf. on Optical Fiber Communication(OFC '89), THJ2, 1989, and H. Taya, K. Ito, T. Yamada and M. Yoshinuma,“Fusion splicer for polarization maintaining single mode fiber”,Fujikura Technical Review, pp. 31-36, 1990, and the fiber end-viewmethod, see U.S. Pat. No. 5,147,434, Sep. 15, 1992, for K. Itoh, T.Yamada, T. Onodera, M. Yoshinuma and Y. Kato, “Apparatus for fusionsplicing a pair of polarization maintaining optical fibers”, and U.S.Pat. No. 5,156,663, Oct. 20, 1992, for K. Itoh, T. Yamada, T. Onodera,M. Yoshinuma and Y. Kato, “Apparatus for fusion splicing a pair ofpolarization maintaining optical fibers”. More advanced techniques, seeWenxin Zheng, “Automated Fusion-Splicing of Polarization MaintainingFibers”, IEEE J. Lightwave Tech., Vol. 15, No. 1, 1997, e.g. thecombination of the polarization observation by lens effect tracing(POL)-profile with the method of POL-correlation for directly andindirectly determining the angular offset of PM fibers, have also beendisclosed, see Swedish Patent No. 9300522-1, March, 1993, inventorsWenxin Zheng et al., U.S. Pat. No. 5,572,313, Nov. 5, 1996, for WenxinZheng et al., U.S. Pat. No. 5,758,000, May 26, 1998, for Wenxin Zheng etal., and the published International Patent application No. WO 01/8633for Wei-Ping Huang et al. These techniques were very successfullyemployed in automated arc fusion splicers for the most common PM fibersthen available in the market, e.g. the Panda and the Bowtie fibers.

Recently, elliptical-core fibers have attracted great interest inconstruction of communication systems, e.g. in constructing erbium-dopedPM fiber amplifiers and optical fiber sensors. Unfortunately, theexisting alignment techniques, see the above-cited patents onPOL-profile methods, can hardly generate stable and consistent resultsof angular alignment for the elliptical-core type due to primarytechnical limitations. For example, the methods are not sensitive enoughto accurately measure the small variations in the intensity profileswhen rotating the fibers. Thus, there is a need in the art to improvethe existing alignment techniques, in particular those based on thePOL-profile, in order to be capable of handling all types of PM fibers.

In particular, these problems appear in illuminating each fiber from aside thereof and regarding the fiber as a cylindrical lens, observingthe light intensity variations in the focal plane along a lineperpendicular both to the longitudinal axis of the fiber and to thepropagation direction of the illuminating light source. Typically, theintensity has a central peak that varies in height when the fiber isrotated about its longitudinal axis, see the Swedish Patent No.9300522-1 and the published International Patent application No. WO01/8633 cited above. In this context it is interesting to calculate thelight contrast, h, which is the difference in intensity between thecentral peak and the surrounding region. The profile of the lightcontrast is obtained as the variation of the light contrast as afunction of the angle of rotation, i.e. the azimuthal angle.

A highest possible contrast of h-values, i.e. of the difference betweenthe maximum and the minimum h values in the profile of the lightcontrast, is essential to ensure a high quality of the contrast profile.It turns out that, for PM fibers of elliptical core type the contrast ofthe h-values is usually less than 10 grey scale levels as measured in atypical automated fusion splicer. Thus in this case, the light contrastprofile becomes extremely sensitive to the adjustment of the opticalimage system of the splicer used.

SUMMARY

It is an object of the invention to provide a method and a device forimproving the quality of the optical measurement of h values for PMfibers and in particular of the contrast in measured POL-profiles.

Thus generally, a careful adjustment of the plane in which the contrastis observed, i.e. of the focal plane, is made.

The adjustment of said plane is done on observing the light intensityvariations in the plane in order to achieve a sharpest or highestpossible contrast. This sharper contrast results in a much betterestimation of the angular position of the principal optical axes of PMfibers using the POL method, this estimation method being suited toalign PM fibers of all types, especially PM fibers having ellipticalcores.

A solution to the above problem may thus be achieved by observing thatthe focal distance of a PM fiber illuminated from the side thereofvaries slightly according to the asymmetry in optical transparency thatappears during rotation of the fiber about the longitudinal axisthereof. A “best match plane”, in which the highest contrast of h-valuescan be obtained, should generally result when the observation planematches the focal plane, and in particular for angular positions of theconsidered fiber for which the slow optical axis of the fiber also isapproximately oriented along the propagation direction of theilluminating light-rays and the optical axis of the image system, thisdirection and the axis being assumed to be parallel to each other. It isfound that the accepted error in determining the position of the bestmatch plane along the light-ray direction depends strongly on the PMfiber considered. For PM fibers having elliptical cores, the acceptableposition error is typically only a few μm.

The method proposed herein improves the processes disclosed in SwedishPatent No. 9300522-1 and the published International Patent applicationNo. WO 01/8633 cited above, by employing a so called “auto-defocusing”method to automatically search for the best match plane. The advantageof such a process is that it yields a better precision in aligning PMfibers having elliptical cores for optimum positions during e.g. afusion splicing procedure.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe methods, processes, instrumentalities and combinations particularlypointed out in, the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth withparticularly in the appended claims, a complete understanding of theinvention, both as to organization and content, and of the above andother features thereof may be gained from and the invention will bebetter appreciated from a consideration of the following detaileddescription of non-limiting embodiments presented hereinbelow withreference to the accompanying drawings, in which:

FIGS. 1 a, b are schematic views of an apparatus for alignment of two PMfibers,

FIG. 2 is a schematic diagram showing the significant difference inlight contrast h for two orientations of a Panda fiber,

FIG. 3 is a diagram showing a typical light contrast profile obtainedfor a Panda fiber,

FIG. 4 is a schematic view of major types of PM fibers, in which theprincipal optical axes for the fibers are marked correspondingly,

FIG. 5 is a diagram showing a typical light contrast profile obtainedfor a PM fiber having an elliptical core,

FIG. 6 is a schematic view of an optical imaging apparatus for searchingthe best match plane in an auto-defocusing application,

FIGS. 7 a, b are images taken from two defocusing directions, some tensof μm away on either side of a reference plane, and

FIG. 8 is a program flow chart illustrating the method of alignment infusion splicing with the aid of auto-defocusing.

DETAILED DESCRIPTION

The basic components of an automatic optical fiber splicer adapted forsplicing PM fibers are shown in the schematic diagrams of FIGS. 1 a and1 b. FIG. 1 a is a schematic of the optical arrangement near the ends oftwo PM fibers 1 and 1′, which are to be fusion spliced to each otherusing an electrical discharge between points of two electrodes 3. Twolight sources 11 illuminate the splice position of the fibers from thesides thereof by parallel light rays in two perpendicular directions.Since an optical fiber itself works as a cylindrical lens, a varyinglight intensity distribution appears in the focal plane of the fiber, asfor instance observed along a line located in the focal plane andlocated perpendicularly to the longitudinal axis of the optical fiber.By using suitable focusing assemblies illustrated by optical lenses 7,four light intensity distributions, as taken in the object planes of thelens assemblies, one for each fiber end and for each direction, are inturn imaged onto the light sensitive surfaces of TV cameras 9,comprising e.g. plates carrying CCD elements. Each picture captured bythe cameras thus comprises images of the ends of the two PM fibers atthe splicing position. The TV signals corresponding to each capturedpicture are then processed in a video interface 31 to convert theintensity values of the pictures to a suitable format for furtherprocessing in an image processing and analysis unit 15 and subsequentpresentation on a video monitor 17.

In FIG. 1 b some more details of the mechanical and electricalarrangements of the fiber splicer are shown. The end portions of the PMfibers 1 and 1′ are thus firmly kept in place by rotating fixtures 22carried by retainers 21 during the alignment and splicing operations. Bymotors 23, the retainers 21 are displaceable in three orthogonaldirections, x, y and z, the z-direction e.g. being taken to be parallelto the longitudinal direction of the fiber ends, see FIG. 1 a, and therotating fixtures are rotatable over an angle of 360°. To the electrodes3 and the motors 23 electric lines extend from driver circuits 27 and29, respectively, in an electric circuit module 25. The TV camera 9 iselectrically connected to a video interface 31 in the electric circuitmodule that is in turn connected to the image processing and analysisunit 15 for delivering captured image information. The various steps forthe processing of input data are controlled by a processor logic circuit33 in the electric circuit module 25 providing control signals to thedriving circuits 27 and 29 in accordance with output data of the imageprocessing and analysis unit 15. That is, as the result of a particularanalysis of the distribution of light intensity in the focal planes ofthe PM fibers 1 and 1′, the fiber ends are displaced in relation to eachother according to given processing algorithms. At the same timeinformation is by the processor logic circuits 33 provided to the imageprocessing and analysis unit 15 for controlling the processing andanalysis performed therein. Also, the processor logic unit 33 controlsthe times when an electric high voltage is to be provided to theelectrodes 3, thus starting an electric current to flow in the dischargebetween the points of the electrodes, and it also controls the durationof this current and the intensity of the current. The processor logicunit 33 also controls the illumination of the light sources 11.

From the images captured by the TV cameras 9 the light intensitydistributions along suitably chosen lines perpendicular to thelongitudinal direction of the ends of the fibers 1, 1′ in the focalplanes of fibers 1 and 1′ are determined by the image processing andanalysis unit 15. From these determined intensity distributions thelight contrast h for each line is calculated by the image processing andanalysis unit 15 as the difference between the maximum intensity in thecentral peak of the respective distribution, this peak corresponding tothe longitudinal center line of the image of the fiber ends, and therelatively constant light intensity of regions of the distributionlocated next to and on the two sides of the peak. It is observed thatthe h-value varies when rotating the respective fiber end about thelongitudinal axis thereof. The variation results from the opticalasymmetry of PM fibers such as stress zones and/or refractive indexdifferences that have been introduced in the cladding and/or core ofsuch fibers. The lack of optical symmetry results in a significantdifference in h-value from one azimuthal angle or angular position toanother one. The h-value can thereby indicate the position of theoptical asymmetries of the fiber end. The h-values determined from thecorresponding intensity distributions when rotating the PM fiber ends,e.g. over a full turn, give a light contrast profile as a function ofthe angular position from which it may be possible to determine theangular orientation of the optical asymmetries and therefrom thepolarization axes of the respective end of a PM fiber. FIG. 2illustrates significantly different h-values obtained for a Panda PMfiber at two unique orientations, i.e. along the slow and fast opticalpolarization axes, respectively.

FIG. 3 is a diagram showing a light contrast profile measured for atypical Panda PM fiber, i.e. the h-values as a function of the rotationangle. It is observed that a two-peak structure appears periodically inthe profile. The periodicity is directly correlated to the symmetry ofthe fiber with respect to the longitudinal axis of the fiber. The mainpeaks in the profile indicate the angular position of the two opticalpolarization axes, i.e. the fast and the slow axis, of the fiber end.The center position of the highest peak in the profile directlyindicates the angular position of the slow axis of the considered PMfiber. This results from the fact that the light contrast graduallyincreases with decreasing angular offset between the propagationdirection of light rays and the slow optical axis. This phenomenon istrue for PM fibers of most types, e.g. Panda and Bowtie fibers and PMfibers having elliptical jackets or elliptical cores. The principaloptical axes for these PM fibers are shown in the cross-sectional viewsof FIG. 4.

The angular orientation of the polarization axes of PM fibers can bedetermined as the positions of the slow optical axes, as given byprofile analysis described above, in relation to some arbitrary zerovalue of the azimuthal or rotation angle. The zero value may be selectedto be e.g. the initial calibration or zero position of the fiber rotaryfixtures 22. The angular positions of the fast optical axes are then 90°away from the angular positions of the slow optical axes.

From a close inspection of FIG. 3, a detailed structure is seen in thehighest peak. This structure can mainly be attributed to the influenceof the fiber core on the light transmission across the stress zones, ata somewhat small range around the angular position of the consideredfiber end in which the slow optical axis is parallel to the direction ofobservation. Therefore, it is necessary to select a suitable methodmodelling the light contrast profile and extracting more detailedinformation from the profile so that the position of the slow opticalaxis can be accurately determined.

In the diagram of FIG. 5 a typical light contrast profile measured for aPM fiber having an elliptical core is plotted. Significant differencesbetween this profile and that measured for the Panda fiber, see FIG. 3,can be observed. First, for a PM fiber having an elliptical core, onlyone peak periodically appears in the profile, this peak indicating theslow optical axis of the fiber. Second, the range of the h-values is oneorder of magnitude smaller, actually around 10 grey scale levels in thecase of PM fibers having an elliptical core as measured in a typicalautomatic fiber splicer, compared to that obtained for the Panda fiberwhich is around 100 grey scale levels. This small range results from thefact that the difference in length between the major and minorelliptical axes of the core in PM fibers having an elliptical core isvery small, only 1-2 μm in a typical case. It is obvious from generalexperience in the field of measurements that a profile having a lowvariation range will often give a low accuracy of the determined angularpositions of the polarization axes and thereby a low quality of theangular alignment that is based on the determined angular positions.Therefore, special techniques, like “auto-defocusing” to be describedhereinafter, have to be employed to overcome this problem.

For the angular alignment of two PM fiber ends, general steps can beperformed as follows: select a well-defined sampling range of angles,typically 360°, in which the light intensity distribution is measuredand therefrom calculate light contrast profiles for the two PM fiberends, find the slow optical axes by a careful analysis of obtainedprofiles, and align the slow axes of the two PM fiber ends with eachother. For the numerical calculations necessary in executing the method,it may be assumed that the total number N of measured points forconstruction of the light contrast profiles of the two fibers can berepresented by two vectors, {Θ_(A), Θ_(B)} respectively:Θ_(A)={θ_(a 0), θ_(a 1), θ_(a 2), . . . , θ_(a axis), . . . ,θ_(a N-1)}  (1)Θ_(B)={θ_(b 0), θ_(b 1), θ_(b 2), . . . , θ_(b axis), . . . ,θ_(b N-1)}  (2)where θ_(a 0) and θ_(b 0) are the initial azimuthal angles with respectto the zero values. If the basically arbitrary, initial positions of thefiber rotators 22 are-selected as the zero values, θ_(a 0)=θ_(b 0)=0°.θ_(a, axis) and θ_(b,) axis are the azimuthal angles where the slowoptical axes are located, i.e. at these angles the slow optical axes areparallel with the respective observation direction which is in theoptical axis of the respective lens assembly 7. Thus, the angular offsetΔΘ between the polarization axes of the two fiber ends is given by:ΔΘ=|θ_(a axis)−θ_(b axis)|  (3)

It should be observed that neither of the true values θ_(a axis) norθ_(b axis) normally is an angle at which a light intensity distributionis measured but these angular position are usually located between twoangular positions at which measurements are made, the measurements beingmade for angles having a finite, constant spacing. The accuracy in thedetermination of the angles θ_(a axis) and θ_(b axis) mainly depends onthe short range variation of light contrast profiles, the selection ofthe model used in the evaluations, as will be described hereinafter, andthe quality of the mechanical rotators 22 used. Although the accuracy ofangular alignment of the polarization axes is not directly determined bythe number N of measured values, it is preferable to take a relativelylarge number of N, typically 90-180 points within a range of 0-180° orof 0-360°, depending on the symmetry of the fibers and requirements onthe resolution of the profile.

After the fusion splicing has been carried out, the remaining angularoffset Δ

between the two slow axes of the two PM fiber ends can be found bydetecting the slow axes using the same method as discussed above,rotating the two spliced fiber ends as one unit. The offset Δ

can be used to estimate the degradation of polarization extinctionratio, PER, due to the splicing operation. This degradation is denotedby ΔΓ and is given by:ΔΓ=Γ_(before)−Γ_(after)  (4)Γ_(after)=10 log {(1+ρ cos2 Δ

)/(1−ρ cos2 Δ

)}  (5)ρ=(10^(|Γ)before ^(|/10)−1)/(10^(|Γ)before ^(|/10)+1)  (6)where Γ_(before) and Γ_(after) are the PERs just before and just afterthe splicing operation has taken place, respectively. The Γ_(before) andΓ_(after) are usually obtained from the measured PER of a highlypolarized light source, taken at the far ends of the first fiber 1 andthe second fiber 1′, respectively. In equations (5) and (6) it is alsoassumed that a short piece of the second fiber 1′, typically 2 meters inlength, should be used to perform measurements of Γ_(after) so that thedegradation of PER caused by the splice can be isolated.

In order to accurately determine the location of the slow optical axisfrom the light contrast profile a curve fitting process, a so-calledChi-Square (χ²) fitting, can be used. Within the frame of this method,it should be possible to model the measured profile by the superpositionof analytic functions plus a noise background. Thus, the quality ofcurve fitting can be evaluated by a reduced Chi-Square (χ²) function.The reduced χ² function can be expressed by: $\begin{matrix}{\chi^{2} = {\frac{1}{N - \mu}{\sum\limits_{i = 1}^{N}\quad\left( \frac{{F\left( \vartheta_{i} \right)} - {\sum\limits_{j = 1}^{n}\quad{G\left( {{\vartheta_{i};a_{1,j}},a_{2,j},{a_{3,j}\ldots}}\quad \right)}} - C}{\Delta\quad F_{i}} \right)^{2}}}} & (7)\end{matrix}$where G(

_(i); α_(1j), α_(2j), α_(3j), . . . ) is an analytic function with thej-th fitting parameters α_(1j), α_(2j), α_(j3), . . . and F(

_(i)) is the average of the i-th measured h-value at the azimuthal angle

_(i) with a measurement error-bar ΔF_(i). Here, the ΔF_(i) is estimatedby the standard deviation: ΔF_(i)≈$\sqrt{\frac{1}{M}{\sum\limits_{l = 1}^{M}\quad\left\lbrack {{F\left( \vartheta_{i} \right)} - {F_{1}\left( \vartheta_{i} \right)}} \right\rbrack^{2}}},$where F_(l)(

_(i)) is the l-th individually measured h-value of the total values of Munities, at the angle

_(i) and N is the total number of h-values, or the total number ofmeasurement points in azimuth. C is a constant value that represents thenoise background of the imaging system. μ is the number of fittingparameters varied during the fitting procedure and n is the number ofindependent analytic functions used in the fitting procedure.

In the profile analysis, it turns out that a single Gaussian functioncan be a suitable analytic function for modelling the highest peak wherethe slow optical axis is located. Thus, the equation (7) can be reducedto: $\begin{matrix}{\chi^{2} = {\frac{1}{N - \mu}{\sum\limits_{i = 1}^{N}\quad\left( \frac{{F\left( \vartheta_{i} \right)} - \underset{j = 1}{\overset{n}{\sum -}}\quad{G\left( {{\vartheta_{i};a_{1}},a_{2},a_{3}} \right)} - C}{\Delta\quad F_{i}} \right)^{2}}}} & (8)\end{matrix}$where G(

_(i); α₁, α₂, α₃) is the Gaussian function with fitting parameters α₁,α₂ and α₃. The parameter α₁ stands for the height of the function, andthe parameters α₂ and α₃ stand for the expected center position of thehighest peak in the profile and the full width to half maximum (FWHM) ofthe peak, respectively. The best set of fitting parameters {α_(1,best),α_(2,best), α_(3,best), C} is the one that maximizes the probability ofthe Gaussian function representing the measured data of the function F(

_(i)). Practically, the parameters {α_(1,best), α_(2,best), α_(3,best),C} are input to a numerical iteration loop in which χ² is calculated. Tosucceed in the fitting process the parameters are then changed in asystematic way in order to achieve a desired result of χ²≈1. Thus, theposition of the slow optical axis will be given by α_(2,best). Theinitial values {α_(1,0), α_(2,0), α_(3,0), C} for fitting are determinedby a pre-analysis of the height profiles, and e.g. α_(1,0)=Max[F(

_(i))], α_(2,0)=

_(i){Max[F(

_(i))]}, α_(3,0)=2|α_(2,0)−

_(k){Max[F(

_(i))]/2}|, C=Min[F(

_(i))] can be chosen.

According to basic mathematics, the light contrast profile can inprinciple be represented by any set of elementary functions, e.g.polynomial and rational functions, logarithmic, exponential, power andhyperbolic functions, trigonometric and inverse trigonometric functions,etc. The selection of the analytic functions depends mainly on therequirements on the alignment accuracy and the time for executing thenecessary calculations. These requirements may vary depending on thetype of PM fiber considered. One typical example is a truncated Fourierseries that was successfully used for modelling the POL-profiles ofPanda and Bowtie types of PM-fibers, see Wenxin Zheng, “AutomatedFusion-Splicing of Polarization Maintaining Fibers”, IEEE J. LightwaveTech., Vol. 15, No. 1, 1997.

As is derived from the discussion above, a largest possible range ofvariation of the h-values is essential to ensure a high quality of lightcontrast profile and thereby a high accuracy of the angular alignment ofthe polarization axes. Unfortunately, as has been mentioned above, forPM fibers having elliptical cores, the variation range of the h-valuesis usually less than 10 grey scale levels as measured in a typicalautomated fusion splicer. This makes the profile and in particularcalculations based thereon extremely sensitive to the adjustment of theoptical image system. In FIG. 6 a typical optical image system used foralignment in an automated arc fusion splicer is schematicallyillustrated. In this case, the fiber is illuminated by a light emittingdiode (LED) 60. A diffuser 70 is installed in front of the LED toimprove the illumination. From the diffused light a sharp lightintensity distribution is formed in a focal plane of the fiber end 1, 1′for a given azimuthal angle. However, due to the changing opticaltransmission transversely through the fiber end during azimuthalrotation thereof, the position of the focal plane varies. Thus, “aneffective focal plane” 80 having a certain width W, indicated by a greyfield in FIG. 6, is formed. On the other hand, the observation plane 90of the optical image system, indicated by the thick black line in FIG.6, is usually very sharp, i.e. has a very small depth or thickness, dueto the high quality of the optical system symbolized by the lens 7. Itcan be realized that the best observation plane for the purpose ofdetermining an accurate value of the location of the slow and fast axesof a PM fiber should result when the observation plane approximatelycoincides with the focal plane obtained when the slow optical axis ofthe considered fiber is oriented in the propagation direction of theilluminating light rays and parallel to the optical axis of the imagingsystem 7, see FIG. 6. For this orientation, the slow optical axis canobviously be resolved in a best way and the largest variation range ofthe h-values can be accordingly obtained. Thus, for such a setting thebest quality of the light contrast profile for determining the desiredangles is obtained. It is found that the accepted mismatch distance interms of the position of the best observation plane depends strongly onthe type of PM fiber considered. For PM fibers of the type having anelliptical core, the accepted mismatch distance is typically only a fewμm.

An “auto-defocusing” process will now be described that aims atautomatically searching for the best observation plane. This process canbe divided into five steps: (1) finding an approximate angularorientation of the slow optical axes for the two fiber ends which are tobe fusion spliced to each other and aligning these axes with each otherbased on the findings, (2) finding a reference position of the opticalsystem for starting the process of defocusing, (3) determining thedirection for defocusing, (4) searching for the best observation plane,and (5) finding an improved orientation of the slow optical axis foreach of the two fibers.

The step (1) is straightforwrard and makes use of the numerical processdiscussed above. For step (2), a reference position of the optical orimaging system for the defocusing process can be taken to be theposition or setting of the optical system having an observation plane 90for which a sharp image of the fiber cladding sides is obtained asviewed in the direction of the optical axis of the imaging system 7since the position of such an observation plane at the sides is nearlyindependent of the actual fiber type for fibers having the same claddingdiameter. Alternatively, the reference position can be taken to be theposition in which the central part of the light intensity-distribution,corresponding to the center of the image of the fiber end and the areanext around it, is sharply imaged on the light sensitive area of thecamera 9. This position also varies only slightly for fibers ofdifferent types. In performing step (3), a special procedure of imageanalysis is executed as will be described hereinafter.

In FIGS. 7 a and 7 b two images are shown which are taken for theobservation plane moved in the two opposite defocusing directions, topositions some tens of μm away from the reference position,corresponding to moving the observation plane to the right side as seenin FIG. 7 a, i.e. closer to the imaging system 7, and to the left side,i.e. farther away from the imaging system as seen in FIG. 7 b, withrespect to the effective focal plane or region 80 indicated in FIG. 6.The light intensity distributions, shown by white lines, obtained fromthe images are seen to be significantly different for the two defocusingdirections. Furthermore, it is observed that a best observation planecan only be found when moving the observation plane in the samedirection as that used for capturing FIG. 7 b.

Therefore, a threshold can be set to easily identify the correctsearching direction. After the correct searching direction has beendetermined, numerical iteration loops with a predefined searching rangeand with a predetermined length of the iteration steps are set in theprocedural step (4). A typical searching range and a typical step lengthmay be 20 μm and 0.5 μm, respectively. The calculations performing in aniteration loop will thus be terminated if an acceptable degree ofdeformation of the light contrast profile is found. Whether the degreeof deformation is acceptable is obtained by a comparison between theabsolute deviation Δh_(i) of the h-values and two predefined thresholdvalues h_(c1) and h_(c2). The values h_(c1) and h_(c2) areexperimentally determined quantities indicating two typical types ofdeformations, called flatness and sparking respectively. The absolutedeviation is calculated in the following way: $\begin{matrix}{{{\Delta\quad h_{i}} = {\sum\limits_{k = 1}^{i + p}{{h_{k + 1} - h_{k}}}}},\quad{i = 1},2,\ldots\quad,{N - p - 1}} & (10)\end{matrix}$and it should fulfill the conditionsΔh_(i)≧h_(c1)  (11)Δh_(i)≦h_(c2)  (12)where h_(c1)≦h_(c2). p is a number of steps smaller than N-−1 forchecking the degree of deformation and the preselected value thereofdepends on the slope of the light contrast profile for the PM fiber ofthe type considered. Typically, p has a value in the range of 3-5. Theprocedural step (5) is again straight-forward according to the abovediscussion of the numerical fitting process.

It should be pointed out that the “auto-defocusing” process outlinedabove may not be necessary for PM fibers having a large variation rangeof their h-values, such as for Panda and Bowtie fibers. This resultsfrom the fact that the acceptable mismatch distances for such PM fibersare much larger, at least 2-3 times larger than for PM fibers havingelliptical cores.

Based on the concepts above, a simplified program flow chart comprisingsteps executed in the method of principal optical axis alignment thatincludes an auto-defocusing procedure is shown in FIG. 8. Referring tothis chart and the block 105 therein, the ordinary fusion process startsby cleaning the PM fiber ends and then aligning them linearly, i.e.aligning the outer sides of their claddings or aligning them based onthe sharp focusing at the center of fiber images, by moving theretainers 21 in the respective coordinate directions x, y, and z, seealso FIGS. 1 a and 1 b. In the next step, block 110, the retained fiberends are, by operating the rotary fixtures 22, simultaneously rotated tomove in parallel as a single unit by suitable angular steps, e.g. over afull turn. For each angular step images are captured as imaged by thelens assemblies 7 and received by the TV cameras 9. Then, from thecaptured images, in the image processing and analysis unit 15 the lightintensity distributions along suitably selected lines perpendicular tothe longitudinal axes of the fiber images are determined and recorded.From the determined intensity distributions the contrast profiles, i.e.the h-values as functions of the rotation angle, are derived in the sameunit 15 and curve fits are carried out as described above. Finally, fromthe fitted curves the angular positions of the slow and fast opticalaxes of the PM fiber ends are determined.

In the next block 115 the variation range of the h-values calculated foreach of the two PM fiber ends is determined as the difference betweenthe maximum and minimum of h, (h_(max)−h_(min)). and it is decidedwhether the determined range (h_(max)−h_(min)) for each of the two PM4fibers is larger than or equal to some predetermined value, such as 30in a standard automated splicing apparatus. If the range is decided tobe larger than the predetermined value the auto-defocusing procedure isnot necessary, and then in block 120 alignment of the polarization axesof the two PM fiber ends can be performed, by calculating the angularoffset ΔΘ or difference in angular orientation of the slow optical axesof the PM fiber ends and then rotating the fiber ends with respect toeach other by an angle corresponding to this difference. The ordinaryfusion procedure is then carried out in the final block 125.

If in the block 115 the variation range of the h-values for either fiberis decided to be smaller than the predetermined value such as 30, ablock 130 is executed, in which first the polarization axes of the twoPM fiber ends are roughly angularly aligned with each other, by rotatingthe fiber ends in relation to each other as described above for block120. Then, a reference position of the focusing setting of the imagingsystem is determined based on a sharp focusing either on the fibercladding sides or on the longitudinal central part of images of thefiber ends, as discussed above. The direction in which to start thedefocusing procedure is then found by moving the observation plane to aposition closer to the imaging system 7 and to a position more distantof the imaging system, rotating the fiber ends to have their slowoptical axes located approximately parallel to the optical axis of theimaging system and capturing images for each of the fibers, determininglight intensity distributions along suitable lines as described aboveand evaluating the determined intensity distributions to find theposition in which the intensity distribution has the steepest andsharpest edges. This position is taken to indicate the direction inwhich the observation plane should be moved to obtain an intensitydistribution having the largest variation range. An iteration loop isthen started in the next block 135 by moving the focus, i.e. theobservation plane, from the reference position by one step having apredetermined length in the determined direction. In block 140, the PMfiber ends are simultaneously rotated stepwise, images aresimultaneously taken for each step, light intensity distributions alonglines are determined, the contrast profiles are calculated and theangular positions of the principal optical axes for each fiber end aredetermined, in a way similar to that described above for block 110. Thesteps are taken in an angular interval located symmetrically about theset angular position in which the slow optical axes are approximatelyparallel to the observation direction/optical axis of the imagingsystem. Thereafter, in block 145, the absolute deviation of the valuesof the now determined contrast profiles, i.e. the h-values, isinvestigated, while choosing the number of checking steps p=≦N−2 andcalculating the absolute variations${\Delta\quad h_{i}} = {\sum\limits_{k = 1}^{i + p_{1}}{{h_{k + 1} - h_{k}}}}$for all integers i=1, 2, 3, . . . , N−p−1, such that the range whencalculating the absolute variations includes the maximum and the minimumvalues of the light contrast profiles. In block 150 it is decidedwhether the two conditions are fulfilled, Δh_(i)≧h_(c1) andΔh_(i)≦h_(c2), for all i:s where h_(c1) and h_(c2) are the flatness andsparking criteria, respectively, as discussed above, and h_(c1)≦h_(c2).In the case where it is decided that the Δh_(i) fulfill the conditions,which means that the auto-defocusing procedure has been successful, theprocess continues to the block 120, in which a final alignment of thepolarization axes of the fibers is performed as described above.

If it is decided in the block 150 that the calculated absolute deviationΔh_(i) does not fulfill the conditions, the auto-defocusing procedurecontinues by performing block 135 again, moving the observation plane bythe same step in the same, determined direction of defocusing as hasbeen described above.

While specific embodiments of the invention have been illustrated anddescribed herein, it is realized that numerous additional advantages,modifications and changes will readily occur to those skilled in theart. Therefore, the invention in its broader aspects is not limited tothe specific details, representative devices and illustrated examplesshown and described herein. Accordingly, various modifications may bemade without departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents. It istherefore to be understood that the appended claims are intended tocover all such modifications and changes as fall within a true spiritand scope of the invention.

1-9. (canceled)
 10. A method of aligning the polarization axes of fiberends of two optical polarization maintaining fibers with each other, themethod comprising the successive steps of: placing the fiber ends ateach other with longitudinal axes of the fiber ends aligned with eachother; rotating the fiber ends in repeated angular steps around thelongitudinal axes of the fiber ends to take successive angularpositions, then for each angular step or angular position: capturingimages of the fiber ends as seen in an observation plane; determiningfrom each captured image a light intensity distribution along a lineperpendicular to the longitudinal direction of each of the fiber ends;and determining from the determined light intensity distribution acontrast value h for each fiber end, resulting for each fiber end in alight contrast profile of h-values determined as a function of theangular position; determining from the light contrast profiles theangular positions of the polarization axes of the fiber ends; androtating the fiber ends in relation to each other by an angle equal tothe difference between the angular positions of the polarization axes ofthe fiber ends; wherein, in the step of capturing images of the fiberends as seen in an observation plane, the observation plane is taken sothat the variation ranges, i.e. the differences between maximum and theminimum values, of the resulting light contrast profiles obtain largestpossible values.
 11. The method according to claim 10, wherein whenselecting the observation plane, focusing is first made on exteriorcladding sides of the fiber ends, as seen in the observation direction,so that the observation plane in a reference position passes through thelongitudinal axes of the fiber ends, or focusing is first made to obtainsharp images of areas of the images corresponding to centrallongitudinal areas of the images of the fiber ends, and that then theobservation plane is moved a distance forwards or backwards from thereference position.
 12. The method according to claim 10, wherein whenselecting the observation plane, the following steps are performed:moving the observation plane to a reference position where a sharp focusis obtained of exterior cladding sides of the fiber ends, seen in theobservation direction, so that the observation plane in the referenceposition passes through the longitudinal axes of the fiber ends, orwhere focusing is such that sharp images of areas of the imagescorresponding to central longitudinal areas of the images of the fiberends are obtained; and then, from the reference position, making aniterative search by moving the observation plane for determining theobservation plane in which images of the fibers then are captured. 13.The method according to claim 12, wherein the step of making theiterative search comprises the steps of: first, a direction for movementof the observation plane is determined, the direction being eitherforwards or backwards; and second, moving the observation plane inrepeated steps of a predetermined first length from the referenceposition and in the determined direction of movement.
 14. The methodaccording to claim 13, wherein said step of determining the directionfor movement comprises the step of moving the observation plane forwardsand backwards from the reference position by a step of a secondpredetermined length and determining for each position the variation ofthe light contrast profile.
 15. The method according to claim 12,wherein before the iterative search the fiber ends are rotated, usingthe determined angular positions, to reference angular positions inwhich their slow polarization axes are parallel to the observationdirection and then for each step of the iterative search, first thefiber ends are rotated by repeated angular steps about the longitudinalaxes to take angular positions within an interval centered around thereference positions, and the light contrast profiles are determined forthe interval, the search stopped in the case where all the lightcontrast profiles have a suitable variation.
 16. The method according toclaim 15, wherein in determining the variation for the determined lightcontrast profiles of each fiber end, degrees of deformation arecalculated according to${\Delta\quad h_{i}} = {\sum\limits_{k = i}^{i + p}{{h_{k + 1} - h_{k}}}}$i=1, 2, . . . , N−p−1, where N is the number of angular positions withinthe interval, p≦N−2 is a predetermined number of checking steps andh_(k), k=1, 2, . . . , N are the determined light contrast values withinthe interval; and wherein iterative search is stopped when all thedegrees Δh_(i) of deformation are within a prescribed range of values,i.e. h_(c1)≦Δh_(i)≦h_(c2), where h_(c1) and h_(c2) are predeterminedthreshold values, for all values of i.
 17. A device for aligning thepolarization axes of fiber ends of two optical polarization maintainingfibers with each other, the device comprising: two retainer means, eachone arranged to hold an end of an optical fiber and adapted to displaceand rotate the end a full turn about the longitudinal axis of the end;control means connected to the two retainer means for controlling themto align the longitudinal axes of ends of optical fibers held by theretainer means and to move the ends into a close relationship at asplice position and then to rotate the ends; a light source forilluminating by parallel light the ends at the splice position from aside of the ends; a TV camera having a light sensitive surface andproviding video signals; a lens assembly having an optical axis forimaging the splice position taken in an observation plane onto the lightsensitive surface, the TV camera thereby capturing images of the spliceposition; processing and analysis means connected to the control meansand to the TV camera for processing and analyzing video signals receivedfrom the TV camera; the control means adapted to control the retainermeans to rotate the fiber end in repeated angular steps around thelongitudinal axis of the fiber end to take successive angular positions,and then for each angular position to control the processing andanalyzing means to determine from the image captured in each angularposition a light intensity distribution along a line perpendicular tothe longitudinal direction of the fiber end and to determine from thedetermined light intensity distribution a contrast value h, thisresulting for each fiber end in a light contrast profile of h-valuesdetermined as a function of the angular position; the control meansfurther adapted to control the processing and analyzing means todetermine from the determined light contrast profiles the angularpositions of the principal polarization axes of the fiber ends; and thecontrol means further adapted to control the retainer means to rotatethe fibers ends in relation to each other by an angle equal to thedifference between the determined angular positions of the principalsaxes of the fiber ends, the control means are arranged to control theretainer means to move the fiber ends so to make the observation planetake a position so that the variation ranges of the determined lightcontrast profiles obtain largest possible values.
 18. The deviceaccording to claim 17, wherein the processing and analyzing meanscomprise: calculating means for calculating the variation ranges as thedifferences between maximum and minimum values of the determined lightcontrast profiles; comparing means connected to the calculating meansfor comparing the variation ranges to a prescribed value; and decisionmeans connected to the comparing means for deciding if the observationis to be moved to a new position.